Method for internal cooling of cast tubes

ABSTRACT

Continuously cast tubes are internally cooled in that the entire amount of water sprayed upon the tube&#39;&#39;s inner surface could vaporize and escape through the mandrel at a speed below the speed of sound in steam.

[111 3,713,478 Jan. 30, 1973 United States Patent 1 Vogt et al.

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FOREIGN PATENTS OR APPLICATIONS schaft, Dusseldorf, Germany [73] Assignee: Firma 1,099,700 2/l96l Germany..............................l64/281 835,326 Germany...... .......................164/28l [22] Flled: 1971 Primary Examiner-J. Spencer Overholser Assistant Examiner-John E. Roethel Attorney-Smyth, Roston & Pavitt 21 Appl. No.: 205,105

Related U.S. Application Data Division of Ser. No. 826,453, May 21 doned.

, I969, abanat the entire amount of water sprayed upon the tubes inner surface could vaporize and escape through the mandrel at a speed below the speed of soundin steam.

Germany.....................P I7 58 393.6

1 Claim, 4 Drawing Figures SHEET 1 OF 3 E 3%;in? ivyvvvv/vw 22.:51fiIA IZI V/A ll l s H E: I 7|; fl/ ///////M 7 mmmmo nan METHOD FOR INTERNAL COOLING OF CAST TUBES This is a division, of application, Ser. No. 826,453, filed May 21, 1969, now abandoned.

The present invention relates to a method and equipment for internally cooling tubing produced by continuous casting of steel. The equipment includes, in particular, a cooled mandrel traversed by a steam conduit. German printed patent application DAS 1,099,700 discloses equipment for continuous casting of hollow tubing, wherein a cooled hollow mandrel is centrally suspended in the cavity of the casting mold. Cooling water is both fed and withdrawn from the upper end of the hollow mandrel. For additional cooling of the cast tube a water pipe traverses the hollow mandrel. A noz 'zle in the lower end of this pipe sprays cooling water onto the interior surface of the cast tube. As the water contacts the hot, interior wall of the tube, part of the water vaporizes. The resulting steam, as well as the cooling water which has not vaporized, is withdrawn through the hollow interior of the tube, in down direction. However, steam escaping from the system in this manner is actually quite annoying to the personnel operating the equipment and molests them severely. Also, the process of cutting the tube into sections is hampered by the continuous flow of steam and coolant residue.

in accordance with another known equipment as disclosed in German Patent 843,138, a mandrel is provided for cooling cast tubes from the interior, in that cooling water passes through the mandrel and leaves the same through a ring-shaped slot at the bottom, to cool directly the interior surface of the cast tube. A steam pipe is provided in the interior of this mandrel for causing the steam to escape the system along a path other than the cast tube. However, in such an arrangement one cannot exclude the possibility that at least some of the produced steam still passes downwardly through the hollow interior of the cast tube, so that the disadvantages of the cooling device as disclosed in DAS 1,099,700 cannot be completely avoided. This will be even more true if the tube is cast from steel as large amounts of water are needed for such a case and large quantities of steam are produced accordingly.

Still another device has been disclosed in German Pat. 835,326 in accordance with which cooling water leaves the bottom of a mandrel through a slot thereof and is caught in a plate resiliently urged against the surface of the formed tube. The dammed cooling water is then sucked through suitable piping, traversing the mandrel, in up direction. Some air is sucked, likewise, through the gap between the plate and the interior surface of the casttube.

All these known cooling devices have a decisive disadvantage which is exhibited particularly during continuous casting of steel. Should, for any reason, a crack occur in the interior of the cast tube, particularly where not completely solidified, then it is inherent that more water than normally will vaporize. Therefore, it is even possible that some of the still liquidous metal pours through the formed crack and plugs up openings through which the steam is supposed to escape. This will be even more the case if a crack in the tube increases so that considerable quantities of liquidous steel fill the entire interior cross sectional area of the tube. The cast tube is thus suddenly plugged up while coolant still pours in. Therefore, within a short period of time all of the water pouring into the tube will vaporize. If there is little or no possibility for all of the steam to escape through the mandrel, the pressure in the interior of tube and mandrel increases rapidly and an explosion will result. Needless to say, such an explosion may be disastrous for man and equipment.

The present invention relates to a system, as well as equipment for cooling the interior of a cast steel tube when leaving the mold operating in continuous casting configuration. The cooling device is constructed and proportioned such that even in case a crack develops in the tube, as was outlined above, explosions do not occur. In accordance with the invention, it is suggested that the cross sectional area of the steam escape path running through a hollow mandrel and through conduits communicating therewith, is dimensioned so that the available cross sectional area in square millimeters is at least 60 fold the value of the amount of cooling water (in liters per minute) as sprayed onto the interior surface of the cast tube. For different dimensions the figures will differ accordingly as the proportionality factor (here 60) involved in the relation is not dimen sionless, but has the dimension of minute per liter per square millimeter. In accordance with this rule the size of the cross section area of the hollow part of the mandrel as steam withdrawal path is determined on basis of a particular rate of cooling water flow needed to obtain the desired internal cooling of the cast tube.

The rule involved in the invention can also be restated as follows: The cross section of the steam withdrawal path through the mandrel must be such that in case the entire amount of water sprayed onto the interior surface of the cast tube vaporizes, the steam must be susceptible to complete withdrawal at speeds below the speed of sound in steam. in short, the rate of liquid coolant supply must not exceed an outflow rate of the same amount of coolant but in the vaporized state at a subsonic speed. A steam withdrawal rate at a speed which is below the speed of sound in steam was found to be the critical condition for avoiding the development of dangerously excessive pressure in the interior of the mandrel and of the cast tube.

It should be noted that the requirements for the cross section of the interior of the mandrel must bear reasonable relation to the outer diameter of the mandrel which determines the inner diameter of the cast tube. Moreover, the mandrel itself must be subjected to continuous cooling. The following additional features are instrumental in permitting the inner cross section of a water cooled mandrel to be dimensioned in accordance with the invention for a given outer diameter thereof. However, these features have significance by themselves.

It is suggested to provide twin walls for the mandrel, and the space between the two concentrical walls is divided into channels by means of radially extending ridges. Two respective adjacent channels are jointed at v the lower end of the mandrel. One of each pair of channels thus established is a continuation of a feeder line for cooling water, whereas the respective other channel of a pair serves as a return and coolant withdrawal path. The mandrel constructed in accordance with this feature has the advantage that the water cooling system for the mandrel itself occupies very little space, and this, in turn, permits the cross section of the interior of the mandrel to be large enough to meet the aboveidentified requirement as to the cross section of the path for steam withdrawal.

A hollow mandrel constructed in accordance with these features can, therefore, be used wherever a relatively large interior space is required. Moreover, a mandrel constructed in this manner provides smooth guidance for the liquid which cools the mandrel very effectively.

In accordance with a still further feature of the invention, the steam withdrawal path is under control of a suction device. The suction device maintains preferably a below normal pressure in the interior of steam pipe and mandrel. This way, not only steam is removed effectively, but certain quantities of air are likewise sucked out of the interior of the cast tube, so that the entire amount of vaporized water is withdrawn with certainty. Moreover, the sucking out of air increases the overall cooling as provided by the system.

While the specification concludes with claims par ticularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which:

' FIG. 1 illustrates somewhat schematically a cross sectional view of equipment used for continuous casting of a tube;

FIG. 2 illustrates on an enlarged scale a section through a hollow mandrel employed in the equipment shown in FIG. 1 and constructed in accordance with the preferred embodiment of this invention;

FIG. 3 illustrates a cross sectional view, A, B in FIG. 2; and

FIG. 4 illustrates the development of the tubular interior of the hollow mandrel.

Turning now to the detailed description of the drawings, FIG. 1 thereof illustrates continuous casting equipment which includes a water cooled casting mold 1 having a cylindrical cavity. Liquid steel is fed continuously into the mold cavity in a manner known, per se. This part of the continuous casting process is conventional and needs no further description. A hollow mandrel 2 is suspended in and projects centrally into the mold cavity. The mandrel is likewise water cooled. A tube 3 results from the continuous casting process and is formed by cooperation of mold l and mandrel 2. Tube 3 leaves the cavity of mold l at the open bottom thereof while being only partially solidified. Essentially, there is a liquidous core 3a and inner and outer solidified regions 3b of tube 3. The outer solidified region forms adjacent the cooled mold, the inner solidified portion forms adjacent the cooled mandrel. Both solidified regions increase in size in radial direction toward each other and the size of the liquidous core decreases accordingly with distance of the tube from mold and mandrel. At a certain distance from the equipment tube 3 is solidified throughout its extension.

The solidification is enhanced considerably by spraying the tubes surface with water. The equipment may thus include conventional means for spraying the outer surface of tube 3 with water in a manner known, per se; also, there may be rolls for moving the continuously cast tube away from the mold. In order to cool the interior of tube 3 there is provided a pipe 4 for traversing the interior of the hollow mandrel and reaching into the space circumscribed by tube 3. Pipe 4 is provided with nozzles 5. Water is fed into pipe 4 and sprayed by the nozzles 5 toward the inner surface of tube 3, thereby cooling the tube 3 directly from the interior.

Some, most, or even all, of the water thus sprayed vaporizes upon contacting the hot inner surface of the tube. The resulting steam is sucked through the interior space 6 of mandrel 2 and into a steam pipe 7 communicating with the space 6. A suction pump 8, or the like, draws the steam into pipe 7. Actually pump 8 provides a relatively low pressure in pipe 7 so that not only vapor but air as well is sucked from the interior of tube 3. As a representative example, the pressure may be reduced to 100 or 150 mm water column.

FIGS. 2 through 4 illustrate hollow mandrel 2 in greater detail. The mandrel has an outer wall 9 from which the tube 3 is drawn. The mandrel has an inner wall 10 and the ring space between walls 9 and 10 is subdivided by ridges 13 to form channels 11 and 12. Cooling water runs through the channels 11 and 12. The individual channels 11 and 12 are grouped in pairs as represented by the designation. A channel 11 is associated with an adjacent channel 12, in that they are joined in the bottom region of the mandrel to form a closed system. Channel 11 of each pair is connected on the upper end of the mandrel to a water feeder line 15, while the cooling water leaves the respective channels 12 through piping 14 to which all channels 12 are connected.

Passing cooling water through the hollow mandrel constructed in accordance with the invention offers the advantage that the cooling system occupies little space, and there remains a rather large region 6 with a large cross sectional area through which can pass the steam which resulted from vaporization of the water sprayed through nozzles 5 onto the hot inner surface of tube 3. This open region 6 serves generally as an escape and steam removal path and should be dimensioned such that even in case the entire amount of spray water turns into steam, that steam can still flow up at a speed below the speed of sound in steam so that excess pressure is never developed inside of the solidifying tube 3.

The following quantative analysis serves as an explanation of the effectiveness of the inventive arrangement. Assuming that a continuous steel tube is to be cast at dimensions of 480 mm outer diameter and 140 mm inner diameter, using an output rate of one-half ton per minute steel. For secondary cooling the total amount of spray water needed is 0.6 liters water per kilogram steel is internally sprayed through nozzles 5. The internal cooling thus requires a flow of liters per minute through pipe 4, and it is this amount of water which is sprayed onto the interior surface of the case tube.

The cross section of flow region 6 must have dimension so that the vapor equivalent of 75 liters can flow through, in one minute's time and at a speed below the speed of sound in steam. One liter of water when vaporized, produces 1,700 liter steam reduced to normal conditions of zero degrees centigrade and 760 mm mercury pressure. Under the same normalized conditions the speed of sound in steam is about 440 meters per second. These data are to be used in the continuity equation which, restated for the present environment, says that the total volume Q of steam developed per minute by vaporization of Q-liters of water equals the cross sectional area F times the flow speed W of the steam (Q'=F-W). That speed W must remain below the speed of sound W i.e., Q F-W,,. Considering conventional units and the required conversion factors, the relationship F 2 60.Q is obtained, with Q being the amount of water actually expanded and in liters per minute, and F being the cross section of space 6 in square millimeters. For a cooling flow of 75 l/min, the cross section F must be at least equal to 4,500 mm The pipe 4 is comparatively small and its dimensions can, in fact, be neglected, i.e., the space occupied by pipe 4 provides only negligible constriction of the flow path of steam through region 6. It follows then, that the mandrel must have these dimensions. As it was assumed that the tube to be cast had to have an inner diameter of 140 mm, the mandrel must have an outer diameter of 140 mm accordingly. The requirement of an interior cross sectional area of 4,500 mm produces an inner diameter of the mandrel of 76 mm. This, in turn, defines the overall dimensions of the mandrel and particularly of the space available for the internal cooling channels 11 and 12 of the mandrel as outlined above. One can see from these dimensions that an efficient cooling system of the interior of the mandrel is indeed a prerequisite for permitting the hollow part of the mandrel to be dimensioned for large cross section of the steam removal path.

The invention is not limited to the embodiments described above, but all changes and modifications thereof not constituting departures from the spirit and scope of the invention are intended to be included.

We claim:

1. The method of cooling internally a tube formed by continuous casting, by spraying water on the inner surface of the tube, comprising the steps of:

providing a hollow mandrel having an internal crosssection which is, in square millimeters, at least sixty times the numerical value (in liters per minute) of the flow rate of the water, the rate being sufficient to cool the tube as the tube is drawn from the hollow mandrel in a manner exposing the interior surface of the tube below the bottom of the mandrel,

feeding the water through the mandrel external to the internal hollow space thereof; spraying the water onto the interior surface of the drawn tube and below the bottom of the mandrel, whereby some of the water vaporizes upon contacting the inner surface of the tube, and

sucking the developed steam through the bottom of the mandrel, above the sprayed surfaces of the tube so that the steam as developing upon exposure of the spray coolant to the surface flow from the surface in up direction towards and through the bottom opening of the mandrel, and at a suction rate so that the amount of steam developed upon complete vaporization, flows through the hollow space of the mandrel at a speed below the speed of sound. 

1. The method of cooling internally a tube formed by continuous casting, by spraying water on the inner surface of the tube, comprising the steps of: providing a hollow mandrel having an internal cross-section which is, in square millimeters, at least sixty times the numerical value (in liters per minute) of the flow rate of the water, the rate being sufficient to cool the tube as the tube is drawn from the hollow mandrel in a manner exposing the interior surface of the tube below the bottom of the mandrel, feeding the water through the mandrel external to the internal hollow space thereof; spraying the water onto the interior surface of the drawn tube and below the bottom of the mandrel, whereby some of the water vaporizes upon contacting the inner surface of the tube, and sucking the developed steam through the bottom of the mandrel, above the sprayed surfaces of the tube so that the steam as developing upon exposure of the spray coolant to the surface flow from the surface in up direction towards and through the bottom opening of the mandrel, and at a suction rate so that the amount of steam developed upon complete vaporization, flows through the hollow space of the mandrel at a speed below the speed of sound. 